The present invention generally deals with systems and methods for the enhanced extraction of oil and gas from hydrocarbon bearing strata using RF heating to cause increased permeability and in situ pyrolysis.
Extraction of oil from oil shale, or more generally, hydrocarbon bearing strata, is an industrial process for oil production. This process converts kerogen in hydrocarbon bearing strata into oil by pyrolysis, hydrogenation, or thermal dissolution. The resultant oil is used as fuel oil or upgraded to meet refinery feedstock specifications by adding hydrogen and removing sulfur and nitrogen impurities. Kerogen is considered to have been formed by the deposition of plant and animal remains in marine and non-marine environments. Each kerogen deposit is unique. Alteration of this deposited material during subsequent geological periods produced a wide variety of kerogen maturities. Source material and conditions of deposition are the major factors influencing the type of kerogen and hence the amount and quality of oil and/or gas formed.
Extraction of oil from hydrocarbon bearing strata in the past has been performed above ground (ex situ processing) by mining the hydrocarbon bearing strata and then treating it in processing facilities. Newer modern technologies are being used to attempt the processing underground (in situ processing) by applying heat and extracting the oil via oil wells. The quality of the oils from shale are highly dependent on the temperature at which the kerogen is “cooked” either in situ or above ground and generally consists of variable molecular weight organic liquids, gasses and condensates.
In situ technologies heat hydrocarbon bearing strata underground by injecting hot fluids into the rock formation, or by using linear or planar heating sources followed by thermal conduction and convection to distribute heat through the target area. The oil is then recovered through vertical wells drilled into the formation. These technologies are potentially able to extract more oil from a given area of land than conventional ex situ processing technologies, as the wells can reach greater depths than surface mines. Unlike for underground mining, there is no requirement to leave pillars in place to prevent roof collapse, which also equates to more oil and gas from the same volume. They also present an opportunity to recover oil from low-grade deposits where traditional mining techniques would be uneconomical.
An in situ shale retort can be formed by many methods, such as the methods disclosed in U.S. Pat. No. 4,043,598 to Gordon B. French et al. The process can also be practiced on shale oil produced by other methods of retorting. Many of these methods for shale oil production are described in Synthetic Fuels Data Handbook, compiled by Dr. Thomas A. Henrickson, and published by Cameron Engineers. Inc., Denver, Colo. For example, other processes for retorting hydrocarbon bearing strata include those known as the TOSCO, Paraho Direct, Paraho Indirect, N-T-U, and Bureau of Mines, Rock Springs, processes.
The Illinois Institute of Technology developed the concept of hydrocarbon bearing strata volumetric heating using radio waves (radio frequency processing) during the late 1970s. This technology was further developed by Lawrence Livermore National Laboratory. Hydrocarbon bearing strata is heated by vertical electrode arrays. Deeper volumes could be processed at slower heating rates by installations spaced at tens of meters. The concept presumes a radio frequency at which the skin depth is many tens of meters, thereby overcoming the thermal diffusion times needed for conductive heating. Its drawbacks include intensive electrical demand and the possibility that groundwater or char would absorb undue amounts of the energy.
Microwave heating technologies are based on the same principles as radio wave heating, although it is believed that radio wave heating is an improvement over microwave heating because its energy can penetrate farther into the hydrocarbon bearing strata. The microwave heating process was tested by Global Resource Corporation. Electro-Petroleum proposes electrically enhanced oil recovery by the passage of direct current between cathodes in producing wells and anodes located either at the surface or at depth in other wells. The passage of the current through the hydrocarbon bearing strata results in resistive Joule heating.
In many cases, before an in situ retorting process can function, it is necessary to develop techniques to increase the permeability of the hydrocarbon bearing strata. Induced fracturing, the best method of increasing the effective permeability of oil-shale deposits, may be accomplished by hydraulic pressure, high explosives, high-voltage electricity, or heating of the formation, or combinations of two or more of these.
Hydraulic fracturing, or fracking, has played an important role in the development of America's oil and natural gas resources for nearly 60 years. In the U.S., an estimated 35,000 wells are processed with the hydraulic fracturing method; it's estimated that over one million wells have been hydraulically fractured since the first well in the late 1940s. Each well is a little different, and each one offers lessons learned. The oil and natural gas production industry uses these lessons to develop best practices to minimize the environmental and societal impacts associated with development. Studies estimate that up to 80 percent of natural gas wells drilled in the next decade will require hydraulic fracturing to properly complete well setup. Horizontal drilling is a key component in the hydraulic fracturing process.
In a hydraulic fracturing job, “fracturing fluids” or “pumping fluids” consisting primarily of water and sand are injected under high pressure into the producing formation, creating fissures that allow resources to move freely from rock pores where it is trapped. Typically, steel pipe known as surface casing is cemented into place at the uppermost portion of a well for the explicit purpose of protecting the groundwater. The depth of the surface casing is generally determined based on groundwater protection, among other factors. As the well is drilled deeper, additional casing is installed to isolate the formation(s) from which oil or natural gas is to be produced, which further protects groundwater from the producing formations in the well. Casing and cementing are critical parts of the well construction that not only protect any water zones, but are also important to successful oil or natural gas production from hydrocarbon bearing zones. Industry well design practices protect sources of drinking water from the other geologic zone of an oil and natural gas well with multiple layers of impervious rock. While 99.5 percent of the fluids used consist of water and sand, some chemicals are added to improve the flow. The composition of the chemical mixes varies from well to well.
Hydraulic fracturing has been successful at increasing the flow of gas from low permeability shales. Low permeability shales are those in which the permeability is than 1 microdarcy and oil and gas cannot be recovered economically without well stimulation. This wet fracturing has been shown to increase the amount of flow from a well many times over by causing cracks in the shale to expose large areas of gas to harvesting. One issue with hydraulic fracturing is that it requires the use of large amounts water at high pressure to cause fractures in the underground hydrocarbon bearing shale. This water is mixed with various chemicals, an individual proprietary mixture for each fracking company, to help with the fracturing process. The amount of water used per well varies by location but can be as much as 4 to 5 million gallons. Getting this much water to the well head can cause wear problems for local roads due to the 400 to 500 heavy tanker trucks required. Pumping this water can cause significant level reduction in local aquifers, which can cause local water wells to run dry. In addition, 10% to 40% of this water comes back to the surface contaminated with subsurface chemicals and needs to be cleaned up before release into the environment, or disposed of in some other environmentally responsible manner. The amount of water required to open up hydrocarbon seal shales has been called the single biggest problem in the shale gas industry
What is needed is a system and method, which can recover the oil and gas in place from subsurface low permeability hydrocarbon bearing strata with minimal water usage. Further, the system and method should also be capable of converting the kerogen within the hydrocarbon bearing strata into additional oil and gas, which can also recovered.